The cells in multicellular organisms and assemblies effect exchanges only through predetermined paths. Communication over short paths is effected so far as known via gap junctions, plasmodesmata or synaptic connections (chemical synapses).
Gap Junctions have protein pores of connexin proteins having a pore size of about 1.5 nanometres. Gap Junctions do not provide a membrane continuum between the cells, but connect neighbouring cells over a distance of 2 to 4 nanometers through the passing on of electrical signals and the passive exchange of small molecules having a molecular weight up to 1000 Dalton, in exceptional cases up to 5000 Dalton. The signal flow is regulated by means of the calcium concentration and/or the applied potential, for which reason gap junctions are also characterized as “electrical synapses”. A transport of membrane vesicles is not described. Gap junctions are found in animal and fungus cells. In plants, only connexin-like proteins have been identified to date.
Plasmodesmata are membrane-surrounded cytoplasma channels. They connect neighbouring plant cells via pores in the cell walls having a diameter of ca. 60 nanometers. This leads to a membrane continuum between the connected cells. The endoplasmic reticuli of connected cells can also extend over plasmodesmata. Plasmodesmata are transparent for small molecules, nutrients, ribonucleic protein complexes, ions and fluorescent dyes. The size rejection limit of plasmodesmata is at circa 1 to 4 kilodalton, but also a few much larger viruses use plasmodesmata as a path for infection. Vesicle transport via plasmodesmata is, however, not known to date.
Synaptic connections are cell continuations (axons) which are connected at their end via a synapse with the surface of another cell. Synapse and target cell are separated by a synaptic gap ca. 20 nanometers wide. The diameter of an axon is ca. 0.3 to 1.3 micrometer. Axons contain microtubules and make possible a bi-directional vesicle transport up to the synapse. Information exchange is effected through the excretion of signal substances at the synapse. These substances diffuse over the synaptic gap to the target cell where through binding to specific receptors they lead to signal transmission. Synaptic connections are characteristic for the central and peripheral nervous system.
Communication or the exchange of information over longer paths is effected, so far as known, via cytonemes, argosomes and via the excretion of messenger substances via the endocrine system.
Cytonemes are fine cell extensions, ca. 200 nanometers thick, which mostly end free in extracellular space, but sometimes also stand in contact with other cells. The extensions contain actin, but no microtubules. At the present time it is not known how cytonemes contribute to intercellular communication. Cytonemes have been detected in insects (Drosophilia, imaginal disk cells), Calpodes, Rhodnius and sea urchin embryos. An occurrence in mammal cells has not to date been described.
Argosomes are membrane vesicles occurring intracellular and extracellular which can transport signal substances from cell to cell. The mechanism of intercellular transport of argosomes has not yet been determined. It is postulated that argosomes are transported via endocytotic and exocytotic mechanisms. Argosomes have to date only been found ii Drosophila embryos.
Endocrine communication between cells distinguishes itself through the excretion of signal substances such as e.g. hormones into the bloodstream, followed by a receptor-mediated effect on the target cells. Endocrine communication is a general principle in organisms.
It is the object of the invention to reveal further forms of interaction, transport and communication between cells. It is in particular the object of the invention to make available pharmaceutical and physical means which can influence communication between cells and intercellular transport. Further it is an object of the invention to make available means and methods for the investigation of the new forms of cell communication and intercellular transport.